1. Field of the Invention
The present invention relates to a pump control system, and more particularly to a system for controlling the operation of either a high-specific-speed turbo pump such as an axial-flow pump or a mixed-flow pump for use in relatively high flow rate and low head applications, or a low-specific-speed pump for use in relatively low flow rate and high head applications, by adjusting the rotational speed of the pump operated by a motor with a frequency/voltage converter (static inverter).
2. Description of the Prior Art
For varying performance characteristics of a pump which is operated by an induction or synchronous motor, there has heretofore been employed a static inverter to vary the frequency of the power supply of the motor to adjust the rotational speed of the pump. To set a rotational speed for the pump, a manual or automatic setting signal is generated by a frequency signal generator within the control range of the inverter which usually ranges from 0% to 120% of the primary frequency of the inverter.
Japanese laid-open patent publication No. 57-52396, for example, discloses an induction motor control apparatus for equalizing the point of intersection between a load torque curve and a motor torque curve to the maximum efficiency point of the motor at a motor input frequency corresponding to the motor torque curve. With the disclosed induction motor control apparatus, the induction motor operates at a maximum efficiency at all times irrespective of the motor input frequency at which the induction motor is energized. Regardless of the rotational speed of a fan coupled to the induction motor, the induction motor can be operated at the maximum efficiency point which corresponds to the motor input frequency at the time.
Another induction motor control apparatus disclosed in Japanese laid-open patent publication No. 59-44997 has a circuit for correcting the output voltage of an inverter depending on the load current of an induction motor so that the output voltage of the inverter reaches a voltage to maximize the efficiency of the induction motor. The disclosed induction motor control apparatus allows the induction motor to be operated highly efficiently irrespective of the operating head of a pump driven by the induction motor, simply by adjusting the primary voltage of the motor depending on the load torque.
Still another induction motor control apparatus has a static inverter for controlling the output power of an induction motor which operates a pump into a constant level, as disclosed in Japanese laid-open patent publication No. 59-25099. Since the motor output power remains constant irrespective of the flow rate Q on a head discharge curve (H.Q curve), the disclosed induction motor control apparatus can lift the H.Q curve to improve operating characteristics of the pump in each of high and low flow-rate regions.
FIGS. 2A through 2C of the accompanying drawings show operating characteristics of a high-specific-speed turbo pump such as an axial-flow pump or a mixed-flow pump for use in relatively high flow rate and low head applications. FIG. 2A illustrates H.Q curves and required power Lp characteristics. Dotted-line curves in FIG. 2A represent characteristics of the pump when the pump is operated by a motor while the frequency of the power supply of the motor is constant. As is well known in the art, when a high-specific-speed pump is operating at a constant power supply frequency, the pump head H sharply decreases in a high flow-rate Q region and increases in a low flow-rate Q region. Therefore, the H.Q curve drops sharply to the right, and the required power Lp also decreases to the right in the graph shown in FIG. 2A. Particularly in the high flow-rate Q region above a rated flow rate, the required power Lp largely decreases as the pump head H decreases.
Stated another way, the marginal power of the motor increases with respect to the motor rated output and the motor does not sufficiently utilize its power in the high flow-rate Q region. If the pump is used as a drainage pump, then when the pump head H decreases, the required power Lp also decreases, making it difficult for the drainage pump to increase the discharged flow rate Q beyond a certain level. Therefore, when the pump head H is low, the drainage pump is required to discharge water for a long period of time. Furthermore, inasmuch as the required power sharply increases in the low flow rate Q region which is about 50% or less of the rated flow rate, if the pump is expected to operate in the low flow-rate Q region, then it is necessary for the motor to have a sufficient rated output power in order to avoid an overload on the motor.
The publications referred to the above disclosed induction motor control apparatuses with various static inverters. However, all of the references fail to disclose an induction motor control apparatus which takes full advantage of the current capacity of the motor that operates the pump. For example, according to Japanese laid-open patent publication No. 59-25099, since the output power of the induction motor is controlled so as to be constant, the voltage V increases and the current I decreases, resulting in a reduced torque while the pump is operating for a low head H and a high flow rate Q. Consequently, there has been a certain limitation to increase the flow rate Q, when the pump is operating for a low head H. The motor cannot be operated fully to its capability by taking full advantage of the full current capacity of the motor.